In recent decades, the field of telecommunications has experienced a significant growth, due primarily to the development of high level digital circuit integration and microprocessors. While microprocessors and associated circuits may not be well adapted for providing the processing capability in large toll or central office switching systems, the integrated circuit microprocessor is well adapted for use in smaller switching systems, such as private automatic branch exchange systems (PABX), key telephone systems, and other types of on premises and office communication systems.
In many office-type switching systems, call processing capabilities must be provided for carrying out the traditional functions such as initiating calls, carrying on and completing telephone calls. One sophisticated microprocessor control complex for such a telecommunications switching system is disclosed in U.S. Pat. No. 4,256,926. By employing a processor control complex, a great deal of flexibility can be provided in making available numerous features to the users of the system. In addition, an all-digital type of system can employ pulse code modulation (PCM) of the audio signals, time division multiplexing (TDM) techniques and microprocessors, all of which are readily available integrated types of circuits for constructing communication systems.
U.S. Pat. No. 4,257,119 discloses a PCM switching system adapted for carrying wide band and narrow band signals. U.S. Pat. No. 4,627,050 discloses a time division multiplex computerized branch exchange system, while U.S. Pat No. 4,627,047 discloses a telecommunication system switching system adapted for carrying both voice and data signals.
Another feature which is provided by many PABX and other types of switching systems is a conferencing capability. That is, a number of parties can be effectively bridged together so as to be able to speak and listen to each other. While conferencing was a relatively easy task when dealing with analog audio signals, the ability to bridge together a number of digital voice channels is more complex. U.S. Pat. No. 4,267,593 discloses a two-port digital conference circuit and the problems attendant with digital conferencing. A multiport conferencing circuit is disclosed in U.S. Pat. No. 4,257,120, in which numerous comparators and registers are required to determine a predominant voice sample to be distributed amongst the conferees.
Another problem attendant with the digital conferencing of a number of voice samples is that of controlling the volume of each channel. Generally, this has been addressed in the analog portions of the voice circuits where manual or automatic volume control can be utilized with relative ease.
In many of the foregoing telecommunication switching systems, and others, the flexibility to expand the system to accommodate additional users is limited. In other words, the growth capabilities are compromised, in that often it is necessary to purchase a system having the initial capability of servicing the number of user ports anticipated to be required in the future. The economic disadvantage of such a system is apparent. In other systems, modules can be added to the switching system as growth requirements dictate, however, certain central processing power must initially exist. The expandability of such type of systems has tradeoffs between initial cost and expansion capability.
It can be seen from the foregoing that a need exists for a communication system in which conferencing is simplified, in which there is not a limitation on the number of simultaneous conferences which can be carried on or the voice quality of the conference, and in which the volume of each channel can be independently controlled in the digital conference circuits.